The international press watchdog, Reporters Without Borders, is to become a joint plaintiff in a complaint by Le Monde alleging that President Sarkozy's aides violated a law protecting the confidentiality of journalists' sources.

It is claimed that government officials used the country's counterintelligence services to identify an Elysée Palace source who leaked information to the newspaper, as Kim Willsher reported last week.

The leaks relate to claims of tax evasion and illegal party funding by the L'Oréal heiress Liliane Bettencourt.

Reporters Without Borders, which is based in Paris, campaigned for years for a law explicitly protecting the confidentiality of journalists' sources, which was eventually adopted.

Now the organisation is outraged that the law appears to have been breached by the president who placed it on the statute book.